1. Field of the Invention
The present invention relates to a high-frequency signal line, and more particularly to a high-frequency signal line preferably for use in high-frequency signal transmission.
2. Description of the Related Art
As a conventional high-frequency signal line, for example, a signal line disclosed in WO2011/007660 is known. The signal line includes a laminate body, a linear signal line and two ground conductors. The laminate body is a laminate of insulating sheets. The linear signal line is provided in the laminate body. The two ground conductors are provided in the laminate body so as to sandwich the linear signal line in the direction of lamination. Accordingly, the linear signal line and the two ground conductors form a triplate-type stripline structure.
Each of the ground conductors has a plurality of openings at positions over the linear signal line when viewed from the direction of lamination. As a result, little capacitance is created between the linear signal line and each of the ground conductors. Therefore, it is possible to reduce the distance in the direction of lamination between the linear signal line and each of the ground conductors, and it is possible to make the signal line thinner. This signal line is used, for example, to connect two circuit boards.
The signal line disclosed in WO2011/007660 has a risk that an attempt to reduce the insertion loss results in breakage of the insulating sheets at the time of manufacture. More specifically, in order to reduce the insertion loss of the signal line, the thickness of the linear signal line shall be increased so that the cross-section area of the linear signal line can be enlarged.
However, the thicker the linear signal line, the more time it takes to complete an etching step for processing a conductive layer into a linear signal line. The etching step is carried out as follows: while each insulating sheet with a conductive layer formed entirely thereon is sent, an etching solution is sprayed on the conductive layer. After the etching step, a pressure-bonding step is carried out as follows: the insulating sheets are pressure-bonded together while the insulating sheets are sent. Accordingly, a reduction in the processing speed at the etching step causes a reduction in the processing speed at the pressure-bonding step, and consequently, the time it takes to manufacture the signal line is increased.
In order to improve the processing speed in the etching step, it is possible that a more acidic etching solution is used. However, the use of a more acidic etching solution may cause damage to the insulating sheets.